When is it proper to use the "—" symbol?

Hi guys, I've seen the — symbol used in various stories. Some use it to extend a sentence, others use it to hold a thought, say something else and finish the thought. Is there a technical term/definition for this symbol and when it should be properly used? Thanks for any help!

it's not a 'symbol'... it's a punctuation mark called the 'em dash'... here is how it's used:

Dash --
Use a dash (represented on a typewriter, a computer with no dashes in the type font, or in a handwritten document by a pair of hyphens with no spaces) . . .

in the following situations:
for example:

to emphasize a point or to set off an explanatory comment; but don't overuse dashes, or they will lose their impact.

To some of you, my proposals may seem radical--even revolutionary.

In terms of public legitimation--that is, in terms of garnering support from state legislators, parents, donors, and university administrators--English departments are primarily places where advanced literacy is taught.

for an appositive phrase that already includes commas.

The boys--Jim, John, and Jeff--left the party early.

As you can see, dashes function in some ways like parentheses (used in pairs to set off a comment within a larger sentence) and in some ways like colons (used to introduce material illustrating or emphasizing the immediately preceding statement). But comments set off with a pair of dashes appear less subordinate to the main sentence than do comments in parentheses. And material introduced after a single dash may be more emphatic and may serve a greater variety of rhetorical purposes than material introduced with a colon.